This invention relates to the field of meters and particularly to meters for continuously measuring the composition and mass flow rate of mixtures containing hydrocarbons. Many types of measurement apparatus have been proposed and are being used for continuously measuring water content, density, or composition of hydrocarbon or hydrocarbon and water mixtures. But most suffer from a number of limitations caused by difficulties with measuring mixtures containing liquid and gas or difficulties associated with variations in the specific gravity of the hydrocarbon being measured. Measuring the composition of oil, water, and gas mixtures is one example. Meters for this purpose are typically referred to as multiphase composition meters. See U.S. Pat. Nos. 4,458,524 and 4,760,742 and U.K. Patent 2,210,461.
The current practice in the oil industry for measuring the quantities of oil, water, and gas being produced by a given well or group of wells is to separate the components in a separator and measure the components individually. The separators are large, expensive, maintenance intensive, and typically provide production information only at long intervals. With continuous multiphase meters to replace the separators, oil producers can dramatically improve the crude oil and natural gas production process, particularly offshore production.
Most proposed multiphase composition meters are designed to continuously measure the volume fractions of oil, water, and gas being produced. The composition meter can be combined with a flowmeter such that production rates for the three components can be calculated. One variety of proposed multiphase composition meter combines a dielectric constant measurement means with a density measurement means. See, for example, U.S. Pat. No. 4,458,524. These devices take advantage of the different dielectric constant and densities of oil, water, and gas respectively to determine their volume fractions. Temperature and pressure sensors are included in the metering package to facilitate these calculations.
In order for them to function properly, they must be able to calculate the dielectric constants and/or densities of the three individual components at the measurement conditions. This is impossible. Several of the lower-density hydrocarbon components (ethane, propane, butane, and pentane among them) can exist in either a liquid or a gaseous state at pressures between 20 and 250 atm. Therefore, the fundamental methods and equations used by these meters to determine the composition of the multiphase production streams are flawed. In fact, it is not possible to accurately determine the volume fractions of oil, water, and gas without knowing how much of each hydrocarbon constituent is in the liquid or gaseous phase at any given time. Such information is not available on a continuous basis.
Another important measurement problem in the oil industry is the accurate measurement of the water content of liquid crude oil streams. The water content directly affects the price paid for crude oil. Many devices have been developed to continuously measure the water content. See U.S. Pat. Nos. 3,498,112 and 4,862,060 for examples. The most commonly used measurement device for this application is a capacitance meter which measures the dielectric constant of the mixture to determine its water content. Many meters besides capacitance meters utilize dielectric measurements for measuring the water content of crude oils, including various microwave meters. A common problem for all of these devices is that the density and dielectric constant of the crude oil vary over time. These variations result directly in significant measurement error.